Late update : Gunman in Mohammad cartoon attack in Texas monitored for years

(Reuters) - Federal agents for years monitored one of the two gunmen who were shot dead after opening fire with assault rifles at a heavily guarded Texas exhibit of caricatures of the Prophet Mohammad.

Two government sources who asked not to be named said the shooters were roommates Elton Simpson and Nadir Soofi, of Phoenix. Court documents show that Simpson had been under surveillance since 2006 and was convicted in 2011 of lying to FBI agents over his desire to join violent jihad in Somalia.

FBI agents and police searched the two men's home at the Autumn Ridge Apartments in north-central Phoenix, cordoning off the complex and evacuating residents for several hours in the early morning.

The shooting incident in the Dallassuburb of Garland was an echo of past attacks or threats in other Western countries against images depicting the Prophet Mohammad.

In January, gunmen killed 12 people in the Paris offices of French satirical magazine Charlie Hebdo in what was said to be revenge for its cartoons

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The Texas incident unfolded on Sunday, when a car drove up behind an indoor arena in Garland, where 200 people were attending an event featuring caricatures of the Prophet Mohammad. Such portrayals are considered offensive by Muslims.

Two men jumped from the car and fired at a police car that was blocking an arena parking lot entrance. A Garland police officer and an unarmed security guard were in the squad car and began to exit as the vehicle approached. The gunmen wounded the security guard, and the police officer returned fire, killing both assailants.

Those inside the Curtis Culwell Center, who had gone through heavy security to enter the event, were not aware of the attack until afterward.

"At this point it does seem clear that an officer of the Garland Police Department acted quickly and decisively and thereby likely saved a number of innocent lives," U.S. Homeland Security Secretary Jeh Johnson said in a statement.